Because a system LSI for a portable terminal, such as a personal digital assistant or a cellular phone, must be capable of long-sustained driving when powered by a battery, it is important to reduce the operating current thereof. In particular, a reduction of the standby current that constitutes an operating current of the device is important especially to determine the standby time of the personal digital assistant when the system LSI is in an idle state. Accordingly, various S-standby current reduction methods have been proposed and developed at present.
To begin with, in accordance with a first method (hereinafter referred to as a software standby), all clocks inside a system LSI are stopped in the standby mode. Because the clock inside the system LSI is stopped when the system LSI enters a standby state, the operating current is set to almost 0 by the circuit operation inside the system LSI. As a result, the standby current can be limited to only the operating current formed by a leakage current. Further, because this method can hold an internal state (such as the value of a register) of the system LSI even when the system LSI is standing by, return processing from a standby state can be performed by interrupt processing. As a result, the return processing is completed only as quickly as necessary for clock restart.
Because the leakage current has been exceedingly high in a miniaturized process in recent years, however, an operating current formed by the leakage current cannot be ignored. Accordingly, Non-Patent Document (Yamada et al., “A 133 MHz 170 mW 10 μA Standby Application Processor for 3G Cellular Phones”, ISSCC 2002, February 6, pp. 370-371) describes a method (hereinafter referred to as a U-standby mode) in which the power of the system LSI is shut off in a standby mode. This method calls for shutting off the power, except for the minimum circuit necessary for recovery processing, when the system enters a standby state. As a result, the operating current caused by the leakage current, as well as the operating current caused by circuit operation, is set to almost 0 and the standby current can be set to almost 0.
The inventors have noticed that the following problems exist as a result of having examined the aforementioned two standby modes.
In the U-standby mode, the operating current formed by a leakage current as well as the operating current resulting from circuit operation, can be set to almost 0. Because the internal state (stored values) of a system LSI is lost by shutting off the power, however, return processing from a standby state cannot be performed by interrupt processing. Accordingly, the return processing must be performed by reset processing. Because reset processing requires initialization and startup of the system LSI, the time necessary for the return processing will be prolonged. In particular, in a software startup, because many instructions are to be executed, their processing time will be prolonged. Specifically, when the system LSI returns from the U-standby mode, the interrupt processing cannot be performed as is even if an interrupt request is posted. However, once the reset processing has been performed, and the software is started up, then the processing that corresponds to the aforementioned interrupt request can be performed.
On the contrary, because the internal state is held in a software standby mode, the software need not be started up. Though a return operation from a standby state is enabled at high speed, the operating current caused by a leakage current will increase as described above.
Thus, the inventor has found that the technology proposed at present makes it difficult to obtain a low standby current compatible with a return operation from a fast standby.